This invention relates generally to screw presses for expressing fluids from fibrous materials, and more particularly to a bi-metallic fully cast worm assembly for use in conjunction with such presses.
The flights on worm assemblies which radially extend from the flight body of feed screws of high pressure expressing presses incur substantial wear and abusive interaction with both fibrous material and debris contained therein as they interact with the walls of the screw press. It is a well-known technique to provide wear resistant or hard-facing coatings upon the surfaces of the flight and flight body which are subjected to highest wear. Techniques utilized for this purpose are deposit welding, flame spray deposition, plasma deposition and the like. Thereafter, the surfaces are smoothed manually back to the desired dimension of the flight. These conventional deposit welding techniques are labor intensive, require expensive components, and provide poor bonding between the ductile base material and the harder deposited weld material.
Considerable effort has been expended to resolve this wear problem as described in the following U.S. and foreign patents known to applicants which include some combination of bi-metallic structure incorporating a tough inner hub portion and a hard or brittle worm flight or portion thereof:
French U.S. Pat. No. 3,592,128 PA1 Bredeson U.S. Pat. No. 3,980,013 PA1 Knuth, et al. U.S. Pat. No. 4,223,601 PA1 Theysohn U.S. Pat. No. 4,364,664 PA1 Mansfield U.S. Pat. No. 4,440,076 PA1 Zies U.S. Pat. No. 3,034,424 PA1 French, et al. U.S. Pat. No. 3,721,184 PA1 Mansfield U.S. Pat. No. 4,838,700 PA1 Williamson U.S. Pat. No. 4,838,700 PA1 --- U.K. 592,834 PA1 --- Italy 557,425 PA1 Appleby U.K. 310,680
Several attempts have also been made to produce a homogeneous feed screw by utilizing casting techniques. However, if a highly wear resistant brittle material is chosen, cracking at the keyway or other highly stressed areas occurs. Alternately, where a more ductile material is used, premature wear of the flight is experienced.
Applicants have also invented another form of a bi-metallic feed screw as described in U.S. Pat. No. 4,996,919. However, this invention is directed to the mechanical engagement of a precast flight within a mating cavity formed within the flight body itself.
The present invention utilizes the techniques of in situ cast forming of the outer worm flight of harder material around the pre-cast inner hub of relatively soft and tough material and a method of manufacture therefor. This structure is ideally suited for high wear resistance, minimum internal stress risers and maximized inner hub toughness and ductility, while also being recyclable.